1. Field of the Invention
The present invention relates broadly to methods and apparatus for filtering solid matter from a gas stream. It particularly relates to improvements in bag house type filters for use in cleaning gas streams.
2. Description of the Prior Art
Various examples of the latter are known including that disclosed in Rolschau's U. S. Pat. No. 3,963,467, dated June 15, 1976. Such dust filter apparatus and others of the prior art, however, suffer major disadvantages, including the problem of redepositing of dust on the filter bags during operation of the apparatus.
It has been established that high velocity fabric filtration has distinct advantages over filtration at low velocities in that the equipment required to clean a gas stream is reduced in size and first cost, as filtration velocity increases. However, experiments on pulse-jet cleaned filters have shown that penetration and pressure drop across the filter media increases markedly at high filtration velocities. The reasons for this include dust redeposition on the bags following a cleaning pulse. At high filtration velocities, most of the dust freed by a cleaning jet pulse is reswept back to the filter bags rapidly and does not fall away therefrom to be collected in the base of the bag house, which is usually of funnel or hopper shape. As a result, a thick dust deposition develops on the filter bags, pressure drop increases, and air flow decreases.
Commercial pulse jet filters are available wherein the gas inlet is located at the bottom of the filter housing. Although a bottom inlet is sometimes preferred, in that it permits large particles in the gas stream to fall immediately into the base-hopper to avoid accumulation on the filter bags, such has the distinct disadvantage of keeping dislodged dust suspended inside the housing and thus permitting easy redeposition on the filter bags.
As may be observed, with bottom entry, the net upward gas velocity within the housing will be greatest at the housing bottom, where the entire gas stream must enter and flow upward to be filtered, and least at the top, by which point all process gas will have been drawn through the bags. Following a cleaning pulse, dislodged dust must fall through the housing bottom, where upward gas velocity is highest. To reach the hopper, particles and agglomerates must have a terminal settling velocity greater than that of the upward moving gas. This limits filtration capacity of the bag house expressed in terms of cubic feet per minute of air per square foot of filter used. Clearly, when small particles or particles which do not agglomerate readily are to be collected in a pulse jet filter, a gas inlet near the housing top is of substantial advantage compared to a gas inlet near the bottom thereof. The advantage becomes greater as filtration velocities increase.
It is reasoned that if redeposition can be minimized in a pulse jet filter, reductions in pressure drop and penetration and increase in air flow will follow.